1. Field of the Invention
This invention relates to decoded-output scanners and, more particularly, to a method and apparatus for optimizing the data throughput of decoded-output scanners by maintaining a tracking relationship between the scanning and decoding operations thereof.
2. Description of the Prior Art
Optical scanners, such as bar code readers, perform the function of converting optically readable patterns into electrical signals. Such patterns may be presented linearly, as in one dimensional (1D) bar code symbols, or two dimensionally as in two dimensional (2D) bar code or other symbols. Decoded-output scanners perform the additional function of interpreting or decoding these signals and obtaining meaningful information therefrom. This is performed by associated decoding circuitry which may or may not be integral to the scanning circuitry. The output of the system, then, is the result of both scanning and decoding.
Optical scanners whether of the stationary or movable type, usually operate at a fixed scanning rate. That is to say the scanners are set so as to perform some fixed number of scans during a given amount of time. This generally ranges between 30 and 200 scans/sec. Prior to the present invention the decoding function of decoded-output readers was synchronized with the scanning function thereof. That is to say that the scanning operation proceeds at a predetermined fixed rate and the information collected by the scanner is decoded by the decoder in a parallel operation.
Prior art optical scanners operate relatively satisfactorily under conditions in which the data throughput rate, or rate at which data is scanned and decoded, is relatively low. If, for example, the scanning rate is relatively low and/or the data content of the bar code or other symbol is relatively small, i.e., the scanner is operating under a relatively light decoding load, the decoding phase of the reading process can be completed between successive scans. Under these conditions scan data can be accurately decoded without difficulty. Operation under these conditions is inefficient, however, since the decoding circuitry is idle much of the time and since operation under such conditions has a low data throughput rate.
Scanners of the above-described type also have the disadvantage that, if they are operated under relatively heavy decoding loads, i.e., are required to scan, at high scanning rates, symbols that have a relatively high data content, the synchronism between the scanning and decoding phases of the reading process will break down. This is because under heavy decoding loads the decoding phase of a read operation takes longer than the scanning phase thereof, causing the decoding operation to lag behind the scanning operation. While this time lag can be dealt with for brief periods of time by storing scan data in a scan memory and decoding that data in the order received when the decoder becomes available, it cannot be dealt with in this way for long. This is because, however large the scan memory, it will eventually overflow and result in a loss of scan data.
Thus, a need has existed for a scanning-decoding method and apparatus that is able to scan and decode optically encoded patterns, such as bar code symbols, quickly and efficiently, even under heavy decoding loads, to ensure that the operator of the scanner obtains the necessary data from the optically encoded patterns.